1. Field of the Invention
The present invention relates to a method of impregnating wood with liquid, and particularly, to a method of efficiently impregnating conifers such as sugi (Japanese cedars) and karamatu (Japanese larches) with liquid containing preservatives, fire retardants, resin, etc., to improve the physical properties such as decay resistance and fire retardancy of the wood.
2. Description of the Related Art
Techniques of infiltrating chemicals such as preservatives and insecticides into wood tissue are widely carried out to suppress the biological deterioration of the wood and improve the durability thereof. Infiltration of fire retardants and resin into wood tissue is also achieved to improve the fire retardancy and water resistance of the wood. These chemicals and resin are dissolved or dispersed in water or organic solvents and are used in a liquid state.
A known method that is practical to efficiently infiltrate liquid into wood tissue is a vacuum-pressure treatment. This method dries wood to be treated up to a proper moisture content, puts the wood in an airtight vessel, reduces pressure in the vessel to remove air from the cells and conducting vessels of the wood, introduces liquid into the pressure-reduced vessel, and pressurizes the liquid so that the wood is impregnated with the liquid.
Since the vacuum-pressure treatment infiltrates pressurized liquid into wood through the fine cells and conducting vessels of the wood from which air has been removed, the liquid receives large resistance from the cells and conducting vessels when it passes therethrough. If the wood is axially large, the liquid takes a long time to penetrate the wood and sometimes hardly reaches the heartwood.
Accordingly, methods are studied to pretreat wood, to change the fine structure thereof so that the wood may easily be impregnated with liquid. One of the methods is a compression-recovery method. This method employs metal molds to compress wood orthogonally to the fiber axis of the wood, thereby promoting the separation of pit covers tori from pits or breaking of closed pits in the conducting vessels of the wood. This may form and expand effective paths for passing liquid in the wood. The compressed wood is transferred from the metal molds into liquid so that the wood may undergo volume relaxation and recover the shape thereof in the liquid. This produces suction force to promote penetration of the liquid into the wood. For example, Mokuzai Gakkaishi (a journal issued by the Japan Wood Research Society) reports in Vol. 41, No. 9, 1995, pp. 811 to 819 that the compression-recovery method expedited liquid penetration on seven kinds of conifers and broadleaf trees.
The compression-recovery method compresses wood in metal molds, cools the wood so that the wood temporarily keeps a compressed state, transfers the wood from the metal molds into liquid, makes the wood recover the shape thereof in the liquid, and employs the recovering force of the wood to infiltrate the liquid into the wood. Consequently, this method needs at least the following six steps of:
1) placing wood to be treated into the metal molds; PA1 2) adjusting the wood to a predetermined temperature; PA1 3) driving the metal molds with, for example, hydraulic pressure to compress the wood; PA1 4) cooling the compressed wood in the metal molds and taking the wood out of the metal molds; PA1 5) immersing the wood, which temporarily keeps the compressed state, in liquid of a predetermined temperature and letting the wood relax the volume thereof and recover the shape thereof; and PA1 6) taking out the liquid-impregnated wood.
In this way, the conventional compression-recovery method needs the step of employing metal molds to compress wood to be treated, the step of temporarily holding the compressed state of the wood, the step of transferring the wood from the metal molds into liquid, etc. Compared with the conventional vacuum-pressure treatment, the compression-recovery method involves such troublesome steps and needs expensive metal molds and an apparatus for driving the metal molds, to increase fixed and manufacturing costs. Accordingly, the compression-recovery method is impractical, and the metal molds easily damage the surface of wood to be treated.